The invention relates to a process for the preparation of dialkylphosphinic acids and salts thereof, and to the use of the dialkylphosphinic acids and salts thereof prepared by this process.
Phosphinic acids and salts thereof can be prepared by various methods and have been described in large number (Houben-Weyl IV, Vol. XII/1, pp. 220 to 240).
The preparation of these compounds is technically complex and is carried out, for example, by oxidation of the corresponding phosphines or via the esters of alkylphosphonous acids, which are themselves prepared from the corresponding phosphonous acid dihalides by reaction with alcohols. The phosphines and phosphonous acid dihalides, for example methyldichlorophosphine, on which the process is based have hitherto been prepared by complex syntheses (Houben-Weyl, Volume 12/1, p. 306). In addition, various by-products are formed which, like some of the abovementioned starting materials too, are toxic, self-igniting and/or corrosive, i.e. are highly undesired.
The organic phosphinic acids and salts thereof based on the abovementioned starting materials, and processes for their preparation and use have in some cases been described.
Thus, for example, the aluminum salts of organic phosphoric acids are known as flame retardants. They can be prepared by various processes.
EP-A-0 299 922 describes a process for the preparation of aluminum salts of phosphoric and phosphonic acid esters by reacting aluminum compounds with phosphoric acid, phosphonic acid or an ester thereof.
In the process described in EP-A-0 245 207, aluminum compounds are reacted with alkylphosphonic acid diesters to give the corresponding aluminum salt.
According to EP-A-0 327 496, the reaction of aluminum hydroxide with alkylphosphonic acid diesters at about 180xc2x0 C. in the absence of water likewise results in aluminum salts of the phosphonic acid monoesters.
EP-A-0 699 708 describes flame-retardant polyester molding compositions in which the polyesters have been rendered flame-resistant by the addition of calcium salts or aluminum salts of phosphinic or diphosphinic acids. The abovementioned salts are obtained by reacting the corresponding dialkylphosphinic acids with calcium hydroxide or aluminum hydroxide.
DE-A-24 47 727 describes flame-retardant polyamide molding compositions which comprise a (metal) salt of a phosphinic acid or of a diphosphinic acid.
However, the abovementioned processes have the disadvantage that the suitable organophosphorus compounds first of all have to be prepared in an inconvenient manner. This is particularly true of the dialkylphosphinic acids, whose aluminum salts are highly suitable for use as flame retardants in certain polymers.
The object of the invention was therefore to provide a process for the preparation of dialkylphosphinic acids and salts thereof in which both the dialkylphosphinic acids themselves and the likewise desired end products, namely dialkylphosphinic acid salts of certain metals, can be prepared in a particularly simple and economical manner.
There is therefore a need for a process for the preparation of dialkylphosphinic acids and salts thereof which can be carried out in a simple manner and in which uniform products are obtained in high yield. A process of this type should also be significantly superior to the processes known hitherto in economic and environmental terms.
This object is achieved by a process of the type mentioned at the outset which comprises
a) reacting elemental yellow phosphorus with alkylating-agents in the presence of a base to give a mixture which comprises, as principal constituents, the (metal) salts of alkyl-phosphonous, phosphorous and hypophosphorous acids,
b) esterifying the principal constituents of the mixture from a) to give an ester mixture,
c) isolating the ester of the alkylphosphonous acid from the ester mixture,
d) preparing the corresponding ester of the dialkyl-phosphinic acid from the ester of the alkylphosphonous acid by free-radical-initiated reaction with olefins,
e) converting the ester of the dialkylphosphinic acid into the free acid or into the dialkylphosphinic acid salts of metals from groups IA, IIA, IIIA, IVA, VA, IIB, IVB, VIIB or VIIIB of the Periodic Table or of cerium.
The process according to the invention has the considerable advantages over the processes known hitherto that it has a positive balance in the product distribution and at the same time avoids the phosphine and phosphonous acid dihalide starting materials, which are regarded as undesired, and in addition produces no halogenated organic by-products.
The alkylating agents are preferably alkyl halides, dialkyl sulfates, trialkyl phosphates, dialkyl carbonates and/or formic acid ortho-esters.
The alkylating agents are particularly preferably methyl chloride, methyl bromide and/or dimethyl sulfate.
The bases are preferably hydroxides, carbonates, bicarbonates, amides, alkoxides and/or amine bases, such as, for example, amines and ammonia.
The reaction in step a) is preferably carried out in a two-phase system comprising aqueous alkali or alkaline-earth metal hydroxide or mixtures thereof and an organic solvent.
The organic solvents employed in step a) are preferably straight-chain or branched alkanes, alkyl-substituted aromatic solvents, water-immiscible or only partially water-miscible alcohols or ethers, alone or in combination with one another.
The organic solvent employed is particularly preferably toluene, alone or in combination with alcohols.
The reaction can, if desired, also be carried out in a non-aqueous system, for example by using solid sodium hydroxide or amines.
The reaction is preferably carried out in the presence of a phase-transfer catalyst.
The phase-transfer catalyst is preferably a tetraalkyl-phosphonium halide, triphenylalkylphosphonium halide or tetraorganylammonium halide.
The temperature during the reaction is preferably from xe2x88x9220 to +80xc2x0 C.
The temperature is particularly preferably from xe2x88x9210 to +30xc2x0 C.
The reaction is preferably carried out under a pressure of from 0 to 10 bar.
Step a) of the process according to the invention is preferably carried out by suspending or dissolving the yellow phosphorus in a solvent or solvent mixture and then reacting it with an alkyl halide and a compound of the formula MOH or Mxe2x80x2(OH)2 or mixtures thereof, where M is an alkali metal and Mxe2x80x2 is an alkaline-earth metal.
The yellow phosphorus and the alkyl halide are preferably reacted with one another in a molar ratio of from 1:1 to 1:3, where the molar ratio of yellow phosphorus to the compound of the formula MOH or Mxe2x80x2(OH)2 is from 1:1 to 1:5.
The principal constituents of the mixture from a) are preferably esterified in step b) using a linear or branched alcohol of the general formula Rxe2x80x94OH, where R is a linear or branched alkyl radical having 1 to 10 carbon atoms.
In another preferred embodiment of the process according to the invention, the principal constituents of the mixture from a) are converted into a mixture of alkylphosphonous, phosphorous and hypophosphorous acids using mineral acids, with the (metal) salts of the mineral acids simultaneously being precipitated, and the mixture of these acids subsequently being esterified.
The water formed during the esterification is preferably removed by azeotropic distillation.
In other words, the esterification of the phosphonous acid to the corresponding monoester can be achieved by reaction with relatively high-boiling alcohols with removal of the resultant water by azeotropic distillation.
The precipitation of the metal salts, usually the alkali or alkaline-earth metal mineral salts, is preferably carried out here by replacement of the solvent water by the alcohol to be used in reaction step b).
The alkali or alkaline-earth metal mineral salt which has already precipitated is preferably filtered off before the esterification.
The alcohol is preferably n- or i-butanol, n-hexanol, ethylhexanol and/or amyl alcohol.
The mineral acid is preferably hydrochloric acid, sulfuric acid and/or phosphoric acid.
The mineral acid is particularly preferably hydrochloric acid.
The phosphines formed in small amounts during step a) are preferably removed by oxidation.
Hydrogen peroxide is preferably used as oxidant.
The ester of the alkylphosphonous acid is preferably removed by distillation in step c). The ester of the alkylphosphonous acid is preferably n-butyl methylphosphonite, isobutyl methylphosphonite, n-hexyl methylphosphonite, 2-ethylhexyl methylphosphonite and/or amyl methylphosphonite.
The free-radical initiators employed are preferably azo compounds or- peroxidic inorganic and/or peroxidic organic free-radical initiators.
The azo compounds are preferably cationic and/or non-cationic azo compounds.
The cationic azo compounds employed are preferably 2,2xe2x80x2-azobis(2-amidinopropane) dihydrochloride or 2,2-azobis(N,Nxe2x80x2-dimethyleneisobutyramidine) dihydrochloride.
The non-cationic azo compounds employed are preferably azobis(isobutyronitrile), 4,4xe2x80x2-azobis(4-cyanopentanoic acid) or 2,2xe2x80x2-azobis(2-methylbutyronitrile).
The peroxidic inorganic free-radical initiators employed are preferably hydrogen peroxide, ammonium peroxodisulfate and/or potassium peroxodisulfate.
The peroxidic organic free-radical initiators employed are preferably dibenzoyl peroxide, di-tert-butyl peroxide and/or peracetic acid.
A broad selection of suitable free-radical initiators is given, for example, in Houben-Weyl, Supplementary Volume 20, in the chapter xe2x80x9cPolymerization by free-radical initiationxe2x80x9d on pages 15-74.
The free-radical initiators are preferably metered in continuously during the reaction.
The free-radical initiators are preferably metered in continuously during the reaction as a solution in the olefin employed.
The olefins employed in step d) are preferably linear or branched xcex1-olefins. The olefins employed are preferably those having an internal double bond, cyclic or open-chain dienes and/or polyenes having 2 to 20 carbon atoms.
The olefins employed are preferably ethylene, n- or i-propylene, n- or i-butene, n- or i-pentene, n- or i-hexene, n- or i-octene, 1-decene, 1,5-cyclooctadiene, 1,3-cyclopentadiene, dicyclopentadiene and/or 2,4,4-trimethylpentene isomer mixtures.
The olefins preferably carry a functional group.
Suitable olefins are compounds of the general formula 
in which R1-R4 may be identical or different and are hydrogen, an alkyl group having 1 to 18 carbon atoms, phenyl, benzyl or an alkyl-substituted aromatic radical.
Likewise suitable are cycloolefins of the formula 
in particular cyclopentene, cyclohexene, cyclooctene and cyclodecene.
It is also possible to employ open-chain dienes of the formula 
in which R5-R10 are identical or different and are hydrogen or a C1- to C6-alkyl group, and R11 is (CH2)n, where n=0 to 6. Preference is given here to butadiene, isoprene and 1,5-hexadiene.
Preferred cyclodienes are 1,3-cyclopentadiene, dicyclopentadiene and 1,5-cyclooctadiene, as well as norbornadiene.
The reaction with the olefins is preferably carried out at a temperature of from 40 to 250xc2x0 C.
This reaction is particularly preferably carried out at a temperature of from 70 to 190xc2x0 C.
This reaction is preferably carried out without a solvent.
Alternatively, it is also possible to carry out the reaction in the presence of a solvent.
The reaction is preferably carried out in an acetic-acid medium.
The reaction is preferably carried out under pressure.
The metals are preferably Li, Na, K, Mg, Ca, Sr, Ba, Al, Ge, Sn, Sb, Bi, Zn, Ti, Zr, Mn, Fe and/or Ce.
In step e), the ester of the dialkylphosphinic acid is preferably reacted directly with metal compounds to give metal salts of the dialkylphosphinic acid.
In step e), the ester of the dialkylphosphinic acid is preferably hydrolyzed to the dialkylphosphinic acid.
In step e), the ester of the dialkylphosphinic acid is preferably hydrolyzed to the dialkylphosphinic acid and subsequently converted into metal salts of the dialkylphosphinic acid.
In step e), the ester of the dialkylphosphinic acid is preferably hydrolyzed to the sodium salt of the dialkylphosphinic acid using sodium hydroxide solution.
In step e), the ester of the dialkylphosphinic acid is preferably hydrolyzed to the sodium salt of the dialkylphosphinic acid using sodium hydroxide solution and subsequently converted into metal salts of the dialkylphosphinic acid.
The reaction in step e) is preferably carried out after an optimum pH range for the salt precipitation has been established for the respective dialkylphosphinic acid/metal compound system.
The metal compounds are preferably metal oxides, hydroxides, oxide hydroxides, sulfates, acetates, chlorides, nitrates and/or alkoxides.
The metal compounds are particularly preferably aluminum hydroxide or aluminum sulfates.
The present invention also relates, in particular, to a process in which yellow phosphorus is reacted with methyl chloride in the presence of sodium hydroxide solution and the phase-transfer catalyst tributylhexadecylphosphonium bromide to give the sodium salt of methylphosphonous acid, the free acid is liberated therefrom by addition of hydrochloric acid, the free acid is esterified in the mixture using 2-ethylhexanol, the ester is isolated by distillation and subsequently reacted with ethylene with free-radical initiation by di-tert-butyl peroxide to give the corresponding ester of methylethylphosphinic acid and finally successively hydrolyzed with sodium hydroxide solution and precipitated as the aluminum salt of methylethylphosphinic acid using aluminum sulfate.
The invention also relates to the use of the dialkylphosphinic acids/salts prepared by the process according to the invention as precursors for chemical synthesis.
The invention also relates to the use of the dialkylphosphinic acids/salts prepared by the process according to the invention for the preparation of other phosphorus-containing compounds and derivatives, in particular as starting materials for crop protection agents.
The invention also relates to the use of the dialkylphosphinic acids/salts prepared by the process according to the invention as flame retardants and as starting materials for the preparation of flame retardants.
The invention relates to the use of the dialkylphosphinic acids/salts prepared by the process according to the invention as starting materials for the preparation of flame retardants for thermoplastic polymers, such as polyethylene terephthalate, polybutylene terephthalate or polyamide.
The invention also relates to the use of the dialkylphosphinic acids/salts prepared by the process according to the invention as starting materials for the preparation of flame retardants for thermosetting resins, such as unsaturated polyester resins, epoxy resins, polyurethanes or acrylates.
Surprisingly, it has been found that elemental yellow phosphorus can, after step a) of the process according to the invention, be reacted with alkylating agents in a two-phase system (organic solvent/base) and, if desired, in the presence of a (phase-transfer) catalyst under extremely mild conditions to give the (metal) salt of the corresponding alkylphosphonous acid RP(:O)HOH.
In addition, small amounts of dialkylphosphinic acids, trialkylphosphine oxide R3P(:O), dialkylphosphine oxide and unidentified phosphorus compounds may be formed; these can be removed from the product mixture in the usual manner. A further by-product formed is hydrogen, which can easily be separated off from the reaction mixture. The abovementioned dialkylphosphinic acids can be separated off from the reaction mixture and employed or further processed elsewhere.
Surprisingly, neither phosphine (PH3) nor alkylphosphines (RPH2, R2PH) are formed in significant amounts in the process according to the invention. Through the choice of suitable reaction conditionsxe2x80x94such as the addition of small amounts of alcohols to the organic phasexe2x80x94the formation of all unidentified phosphorus-containing by-products is minimized to a surprisingly low content of a few mol % of the yellow phosphorus employed, in favor of the main product, the (metal) salts of alkylphosphonous acid.
The process according to the invention can be carried out, for example, by initially introducing the solvent together with the phase-transfer catalyst and, if necessary, warming the mixture to above the melting point of the yellow phosphorus, then adding the elemental (yellow) phosphorus, cooling the mixture to temperatures of, for example, from xe2x88x9210 to +30xc2x0 C. with vigorous stirring, and subsequently adding the alkylating agent.
The reaction is initiated by addition of the base. When the reaction is complete, the reaction system can be diluted, for example with water, and the readily volatile components (H2, PH3, RPH2, R2PH and excess alkylating agent, etc.) are subsequently removed.
This gives a base-containing/organic two-phase system, whose phases are separated. The contents from the phases are determined analytically.
The reactants can also be combined in a different sequence, for example by introducing them continuously into a reactor (pressure tube, pressure reactor or cascade) in the above-defined molar ratio and removing them from the reactor again after a residence time of from 0.5 to 2 hours. The organic phase obtained after phase separation, which still contains the majority of any phase-transfer catalyst employed, is advantageously recycled.
The isolation of the pure alkylphosphonous acids from the mixture is carried out in a particularly simple manner via the corresponding esters, which, in contrast to the salts and acids of the alkylphosphonous acids, can be isolated from the mixture in a gentle manner by distillation. Although all other compounds present in the mixture are also partially esterified in steps b) and c) of the process according to the invention, they do not, however, form readily distillable products, and consequently the removal of the alkylphosphonous acid esters is achieved in surprisingly complete and pure form.